The present invention relates to the field of chemical testing, modifying the surface of a material to promote enhancement of endothelial and osteoblast cells monolayer coverage and sterilization of intermetallic materials. More specifically, it is for testing, surface modifying and sterilization of the electropolished and magnetoelectropolished nickel titanium intermetallic compound known as Nitinol.
The purpose of this test is the detection of Nitinol surface inclusions (except for titanium dioxides TiO2 native inclusions originated during melting process, which do not dissolve in sodium hypochlorite (NaClO) and are harmless to endothelial and osteoblast cells, but still can be the fracture initiation sites), which are indicators of the lack of chemical homogeneousness of the surfaces by immersing electropolished or magnetoelectropolished Nitinol surfaces in an aqueous solution of sodium hypochlorite (NaClO) and checking for black flocculent precipitate developing on the particular surface site.
Simultaneously, the electropolished and magnetoelectropolished Nitinol surfaces, which lack surface inclusions (except titanium dioxide TiO2 native inclusions), undergo chemical modification which leads to better and faster endothelial and osteoblast cells monolayer formation. The surface modification by chemical treatment with sodium hypochlorite (NaClO) promotes endothelial (vascular implantable devices) and osteoblast (orthopedic implantable devices) cell adhesion and proliferation on implantable electropolished and magnetoelectropolished Nitinol devices.
The electropolished and magnetoelectropolished Nitinol surfaces which underwent the sodium hypochlorite treatment without detection of intermetallic surface inclusion (except titanium dioxide TiO2 native inclusions) are sterile and ready for implantation.
The site or sites where this black flocculent precipitate starts to develop is the place of surface inclusion and gives evidence of the lack of homogeneousness of the Nitinol surface, which is a sign of inferiority and should be the base for rejection of such electropolished or magnetoelectropolished Nitinol implantable devices.
The surface inclusions cause maximum stress during bending-rotation and flexing, especially in peripheral stents, and lead to their fracture. Also, inclusions are themselves source sites where corrosion starts to dissolve matrix materials releasing nickel ions harmful to living cells surrounding a particular implantable device.
The Nitinol inclusions can be classified in two ways: by their origin and by their chemical composition. The classification by origin gives two kinds of inclusions: native, which originate during production of bulk material, and foreign, introduced during finishing operations.
The native inclusions are randomly distributed through the whole volume of material and finding them on the surface of a nitinol implantable medical device which underwent electropolishing or magnetoelectropolishing (except titanium dioxides TiO2 native inclusions) should validate rejection of such device. In contrast, foreign inclusions are strictly surface phenomenae introduced to the surface during finishing operations as: glass-bead, sand or aluminum oxide blasting, heat treatment, mechanical polishing, lapping, laser cutting, drawing, electro discharge machining etc.
The classification by chemical composition is more complicated. Taking under consideration the very small size of inclusions the chemical analysis is often difficult and very often leads to errors. Those inclusions could be broadly classified as carbides (TiC), oxides (Ti4N2Ox, TiO2) or intermetallic precipitates (Ni4Ti3).
It is widely recognized that carbides are created during (VIM) vacuum induction melting from carbon crucibles used in this process. On the other hand, oxides are originated in a higher amount and in a larger particle size during (VAM) vacuum arc melting. The third process which claims four to ten times lower carbon content due to use of water cooled crucibles is (EBM) electron beam melting.
Regardless of the above Nitinol production methods, not one of them is perfect and in the present time it isn't possible to produce 100% inclusions free, homogenous Nitinol.
Until now the only ways to check up the Nitinol surface for inclusions were microscopic and instrumental methods as: scanning electron microscope (SEM) with energy dispersive X-ray (EDX) spectrometry, atomic force microscopy (AFM), transmission electron microscopy (TEM) X-ray diffraction, and Auger spectrometer with back-scatter electron detector (BSE).
All of those above mentioned techniques are very expensive, time consuming, demanding highly trained operators and by this they are excellent techniques for scientific research or limited industrial inspections, but unpractical for large scale production inspections.
Because the present invention is very simple, inexpensive, very effective (almost 100%), doesn't require expensive instrumentation and highly trained operators and can be applied to every single electropolished or magnetoelectropolishd implantable medical devices, it is perfectly suited for mass inspections.
The post production test of finished products can eliminate defective products which have avoided detection during raw material testing, because inclusions were not present on the surface during initial test and appear on the surface as a results of production operations: removing excess material by mechanical, chemical or electrochemical processes and revealing inclusions from the bulk of material or by introducing externally new inclusions to the surface of finished products as a result of manufacturing operations as laser cutting, sand blasting, drawing etc.
By applying post production lots testing, very serious (fracture of endodontic rotary file for example) and even life-threatening problems (as for example fracture of carotid stent or heart valve) could be avoided.